In their experimental markets, Colin Camerer, the Robert Kirby Professor of Behavioral Economics at Caltech, and colleagues found two distinct types of activity in the brains of participants—one that made a small fraction of participants nervous and prompted them to sell their experimental shares even as prices were on the rise, and another that was much more common and made traders behave in a greedy way, buying aggressively during the bubble and even after the peak. The lucky few who received the early warning signal got out of the market early, ultimately causing the bubble to burst, and earned the most money. The others displayed what former Federal Reserve chairman Alan Greenspan called "irrational exuberance" and lost their proverbial shirts. The researchers set up a simple experimental market in which they were able to control the fundamental, or actual, value of a traded risky asset. In each of 16 sessions, about 20 participants were told how an on-screen trading market worked and were given 100 units of experimental currency and six shares of the risky asset.